1. Field of the Invention
The present invention relates to spectrometers and particularly to an apparatus for measuring the spectra of a high optical density sample having substantially known optical characteristics.
2. Description of the Prior Art
Optical spectrometers measure the fraction of energy absorbed or attenuated by a sample of material from a beam of optical radiation as a function of the frequency of the radiation. These optical spectrometers are based on a dispersive element, which may be a prism or a diffraction grating, such as a plane or concave diffraction grating. The dispersive element selectively disperses the attenuated radiation from the sample into various spectral components of different wavelengths which, in turn, are focused upon some sort of detection means. The detection means transduces these incident spectral components into some usable form.
Various types of detectors may be used such as, for example, film, photodiode circuits or a multielement detection system. In multielement or parallel detection, the attenuated source radiation from the sample is dispersed rather than wavelength scanned, and the entire spectrum is collected simultaneously by an array of detectors. The simplest such parallel-detection system is the photographic emulsion. Optoelectronic image detectors, such as self-scanned photodiode arrays and multichannel plates, are other examples of parallel detectors. The optoelectronic detector is too subject to blooming (channel-to-channel cross-talk due to overspill of photon-generated charge) and spectral interference (where neighboring features are masked) to be employed in low transmittance applications. Blooming and spectral interference are minimized with a two-dimensional optoelectronic image detector but not sufficiently enough for high optical density applications. In a proposed direct-reader, a nonscanned photomultiplier array is used for detection. However, broadband radiation, instead of discrete radiation, is used for the source. The detection limits and resolution required in high optical density spectroscopy are not attainable in a direct-reader.
It should also be noted that the typical prior art spectrometer ulilizes a lamp, such as an Xenon lamp, for a radiation source. Such a lamp produces optical radiation throughout the entire spectrum. Thus, the optical power from the lamp is spread across the entire spectrum. As a result, the typical prior art spectrometer substantially cannot detect any light passing through a sample which has an optical density greater than 6. In other words, the typical prior art spectrometer can only reliably detect light from a sample that only attenuates the light incident on the sample by no more than 6 orders of magnitude.
Various prior art spectrometers employing a standard scanning spectrometer optical design with increased baffling have been implemented, but optical density limits of only 12 have been achieved.
One prior art scanning spectrometer, that enjoyed a minor amount of success, used low-resolution radiation generated by an Xenon lamp and filtered through a double monochromator to illuminate the high density sample. The attenuated radiation from the sample was collected with a GaAs detector using photon-counting techniques. This technique allowed a broad, low-resolution spectrum of the sample to be determined.
Another prior art scanning spectrometer used a high-resolution, tuned, doubled, flashlamp-pumped-dye laser as the radiation source with the appropriate substitution of pulsed-detection techniques. This technique is not satisfactory for a number of reasons, namely, the system contains a number of moving parts that must constantly be aligned (including the optics required to cover the broad spectral range of the laser tuning), the calibration is difficult and unreliable, and the dynamic range of the system is limited to only 9 orders of magnitude.